The invention relates to a measuring system for measuring a density of a medium variable with respect to a thermodynamic state, especially at least partly compressible, flowing in a process line, such as a process pipeline or tube, along a flow axis of the measuring system. The measuring system measures by means of a temperature sensor, a pressure sensor and a measuring electronics communicating, in each case, at least at times, with the temperature sensor and the pressure sensor, and producing, at least at times, at least one density measured-value representing, as accurately as possible, a local density of the flowing medium.
For registering process-describing, measured variables of flowing media, such as the thermodynamic state variable, density, or measured variables derived therefrom, and for producing measured-values correspondingly representing such measured variables, industrial process measurements technology applies measuring systems installed near to the process. This is done especially also in connection with the automation of chemical processes or processes involving adding value to materials. These measuring systems are often composed of two or more, discrete, measuring, field devices, which communicate with one another and are each arranged directly at, on or in a process line, through which the medium flows. The measured variables to be registered can include, besides density, for example also other thermodynamic state variables, especially such variables as are registerable by sensor and, as a result, directly measurable, such as e.g. pressure or temperature, directly or indirectly measurable flow parameters, such as e.g. a flow velocity, a volume flow, e.g. a volume flow rate, or a mass flow, e.g. a mass flow rate, or other complex transport variables, such as e.g. a heat flux, as well as also other measured variables specific to the medium, such as e.g. a viscosity, of an at least partly liquid, powdered or gaseous medium conveyed in a process line embodied, for example, in the form of a pipeline.
Especially for the indirect (in the following, thus, also referred to as virtual) measurement of density, based on pressure and temperature measurement signals generated by means of corresponding sensors, as well as also measured variables possibly derived therefrom, for example mass flow or volume flow, a large number of industrial standards have become established, which recommend a largely standardized, and, thus, comparable, calculation, especially also with application of directly registered and, thus, actually measured temperatures and/or pressures, and which find application as a function of application area and medium. Examples of such standards include, by way of example, the industrial standard “IAWPS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam”, International Association for the Properties of Water and Steam (IAWPS-IF97), “A.G.A. Manual for the Determination of Supercompressibility Factors for Natural Gas—PAR Research Project NX-19”, American Gas Association (AGA-NX19, Library of Congress No. 63-23358), the international standard ISO 12213:2006, Part 1-3 “Natural gas—Calculation of compression factor”, as well as also the therein cited A.G.A. Compressibility Factors for Natural Gas and Other Related Hydrocarbon Gases”, American Gas Association Transmission Measurement Committee Report No. 8 (AGA-8) and “High Accuracy Compressibility Factor Calculation for Natural Gases and Similar Mixtures by Use of a Truncated Viral Equation”, GERG Technical Monograph TM2 1998 & Fortschritt-Berichte VDI (Progress Reports of the Association of German Engineers), Series 6, No. 231 1989 (SGERG-88).
Often, the ascertaining of density can also serve for converting a directly measured, mass flow into an, as a result, indirectly or virtually measured, volume flow, or vice versa. For direct measurement of flow parameters serving as primary measured variables therefor—thus, for example, a local flow velocity, a local volume flow, or a local mass flow—measuring systems of the type being discussed include at least one corresponding flow sensor, which, by reacting at least predominantly to a flow parameter primarily to be registered for the flowing medium, or also to changes of the same, delivers, during operation, at least one measurement signal, especially an electrical measurement signal, correspondingly influenced by the measured variable primarily to be registered and representing such as accurately as possible. The at least one flow sensor can, in such case, be embodied to contact the medium, at least partially, for example by being immersed therein, or to measure externally through the wall of the process line or a membrane, or diaphragm. Usually, the flow sensor is provided, in such case, by means of a, most often, very complex flow transducer, which is inserted appropriately directly into the process line, or into a bypass, conveying the medium.
Marketed flow transducers are usually implemented as prefabricated, pre-calibrated units equipped with a carrier tube insertable into the course of the pertinent process line and also with at least one physical-to-electrical converting element appropriately pre-assembled therewith. This converting element, possibly in conjunction with the carrier tube itself and/or other components of the flow transducer, especially passive-invasive components, such as e.g. flow obstacles protruding into the flow and/or active components of the flow transducer, such as e.g. a coil arrangement placed externally on the support tube for generating a magnetic field, or sound producing units, forms the at least one flow sensor delivering the measurement signal. Widely distributed in industrial measurements technology are, especially, magneto-inductive flow transducers, flow transducers evaluating the travel time of ultrasonic waves coupled into flowing media, eddy flow transducers, especially vortex flow transducers, flow transducers with oscillating measuring tubes, flow transducers making use of pressure differences, or thermal flow-measuring transducers. Principles of construction and functioning of magneto-inductive flow transducers are described e.g. in EP-A 1 039 269, U.S. Pat. Nos. 6,031,740, 5,540,103, 5,351,554, 4,563,904, while such for ultrasonic flow transducers appear e.g. in U.S. Pat. Nos. 6,397,683, 6,330,831, 6,293,156, 6,189,389, 5,531,124, 5,463,905, 5,131,279, 4,787,252. Since also the others of the aforementioned principles of measurement usually put into practice in industrial flow measuring transducers are likewise sufficiently known to those skilled in the art, a further explanation of these and other principles of measurement established in industrial measurements technology and implemented by means of flow measuring transducers can be omitted here.
Industrial measuring systems registering flow parameters involve, often, those, in the case of which, at least one of the measuring points delivering the actual measurement signals and thus, in the following, referred to as real, is formed by means of a compact inline measuring device having a flow transducer of the aforementioned kind. Further examples for such measuring systems, especially measuring systems formed by means of compact, inline measuring devices with flow transducers known per se to those skilled in the art, are presented, additionally, in detail in, among others, EP-A 605 944, EP-A 984 248, EP-A 1 767 908, GB-A 21 42 725, U.S. Pat. Nos. 4,308,754, 4,420,983, 4,468,971, 4,524,610, 4,716,770, 4,768,384, 5,052,229, 5,052,230, 5,131,279, 5,231,884, 5,359,881, 5,458,005, 5,469,748, 5,687,100, 5,796,011, 5,808,209, 6,003,384, 6,053,054, 6,006,609, 6,352,000, 6,397,683, 6,513,393, 6,644,132, 6,651,513, 6,651,512, 6,880,410, 6,910,387, 6,938,496, 6,988,418, 7,007,556, 7,010,366, US-A 2002/0096208, US-A 2004/0255695, US-A 2005/0092101, US-A 2006/0266127, WO-A 88/02 476, WO-A 88/02 853, WO-A 95/08758, WO-A 95/16 897, WO-A 97/25595, WO-A 97/46851, WO-A 98/43051, WO-A 00/36 379, WO-A 00/14 485, WO-A 01/02816, WO-A 02/086 426, WO-A 04/023081 or WO-A 04/081500, WO-A 05/095902, as well as also in the not pre-published applications DE 102006034296.8 and 102006047815.0 of the assignee.
For the further processing or evaluation of measurement signals produced in the measuring systems, such additionally include at least one corresponding measuring electronics. The measuring electronics, communicating in suitable manner with the pertinent measuring transducer, especially also with the at least one converting element, produces during operation, with application of the at least one measurement signal, repeatedly, at least one measured-value instantaneously representing the measured variable, thus, for example, a mass flow measured-value, volume flow measured-value, a density measured-value, a viscosity measured-value, a pressure measured-value, a temperature measured-value, or the like. The measured-values, especially the indirectly, or also virtually, measured, density measured-value, are, in such case, often ascertained by means of highly complex calculations according to one of the mentioned industry standards, for example “AGA 4”, “AGA 8”, “AGA-NX19, “IAWPS-IF97”, “SGERG-88”, or the like.
For accommodating the measuring electronics, such measuring systems include, most often, a corresponding electronics housing, which, as proposed e.g. in U.S. Pat. No. 6,397,683 or WO-A 00/36 379, can be arranged remotely from the measuring transducer and connected with such via a flexible cable. Alternatively thereto or in supplementation thereof, the electronics housing can, however, also, as shown, for example, in EP-A 903 651 or EP-A 1 008 836, be arranged directly on the measuring transducer or on a measuring transducer housing separately housing the measuring transducer, in order to form a compact, inline measuring device, for example a Coriolis mass flow/density measuring device, an ultrasonic flow-measuring device, a vortex flow-measuring device, a thermal flow-measuring device, a magneto-inductive flow-measuring device, or the like. In the case in which the electronics housing is arranged on a measuring transducer housing, the electronics housing serves, as shown, for example, in EP-A 984 248, U.S. Pat. Nos. 4,716,770 or 6,352,000, often also for accommodating some mechanical components of the measuring transducer, such as e.g. elements deforming during operation on the basis of mechanical effects, elements such as membrane, rod, sleeve or tubular deformation- or vibration-elements; compare, in this connection, also the U.S. Pat. No. 6,352,000 mentioned above.
In the case of measuring systems of the described kind, the measuring electronics is usually electrically connected via electrical lines, and/or wirelessly by radio, with a superordinated, electronic, data processing system arranged, most often, spatially remotely, and also spatially distributed, from the measuring electronics. To this data processing system are forwarded, in near-time, the measured-values produced by the measuring system. The measured-values are forwarded by means of measured-value signals carrying the measured-values. Measuring systems of the described kind are, additionally, usually, by means of a data transmission network (wired- and/or radio-based) provided within the superordinated data processing system, connected together and/or with corresponding electronic process controls, for example programmable logic controllers (PLCs) installed on-site or process control computers installed in a remote control room, where the measured-values produced by means of the measuring system and digitized in suitable manner and correspondingly encoded are sent. By means of process control computers, with application of correspondingly installed software components, the transmitted measured-values can be further processed and visualized as corresponding measurement results e.g. on monitors and/or converted into control signals for other field devices, such as e.g. magnetically operated valves, electric motors, etc., embodied as actuators for process control. Accordingly, the data processing system serves usually also for conditioning the measured-value signal delivered from the measuring electronics corresponding to the requirements of downstream data transmission networks, for example suitably digitizing such and, on occasion, converting it into a corresponding telegram, and/or evaluating it on-site. For such purposes, provided in these data processing systems, electrically coupled with the pertinent connection lines, are evaluating circuits, which pre- or further-process, and, if required, suitably convert, measured-values received from the measuring electronics. Serving for data transmission in such industrial data processing systems, as least sectionally, are, especially serial, fieldbuses, such as e.g. FOUNDATION FIELDBUS, CAN, CAN-OPEN, RACKBUS-RS 485, PROFIBUS, etc. or, for example, also networks based on the ETHERNET standard, as well as the corresponding standardized transmission protocols, which are, most often, independent of application.
Usually, it is possible to implement by means of control computers, besides such process visualization, monitoring and control, also remote servicing, parametering and/or monitoring of the connected measuring system. Accordingly, measuring electronics of modern, measuring, field devices permit, besides actual measured-value transmission, also transmission of various setting- and/or operating-parameters used in the measuring system, such as e.g. calibration data, measured-value ranges and/or also diagnostic values ascertained internally in the field devices. In support of this, operating data intended for the measuring system can, most often, likewise be sent via the aforementioned data transmission networks, which are, most often, hybrid as regards transmission physics and/or transmission logic.
Besides the evaluating circuits required for processing and converting measured-values delivered from connected measuring electronics, superordinated data processing systems of the described kind include, most often, also electrical supply circuits serving for supplying the connected measuring electronics and, as a result, also the pertinent measuring system with electrical energy, or power. The supply circuits provide for the pertinent measuring device electronics an appropriate supply voltage, which is, on occasion, fed directly by the connected fieldbus, and drive the electrical lines connected to the measuring device electronics, as well as the electrical currents flowing therethrough. A supply circuit can, in such case, for example, be assigned to exactly one measuring electronics and accommodated together with the evaluating circuit associated with the particular measuring device, for example joined to form a corresponding fieldbus adapter, in a housing common to both, embodied e.g. as a top-hat rail module. It is, however, also quite usual to accommodate such superordinated evaluating circuits and supply circuits, in each case, in separate housings, on occasion spatially removed from one another and to wire them appropriately together via external cables.
In the case of industrial measuring systems of the type being discussed here, often involved, as a result, are spatially distributed measuring systems, wherein, in each case, a plurality of measured variables of equal and/or different type are locally registered by sensors at real, mutually separated measuring points along a flow axis of the measuring system defined by the process line. These measured variables are fed to the common measuring electronics in the form of corresponding, electrical, measurement signals by wire, for example also in the so-called HART®-MULTIDROP-method or also in the so-called burst-mode method, and/or wirelessly, especially by radio and/or optically, on occasion also encoded into a digital signal or in a digitally transmitted telegram. For the case described above, in which such a measuring system is formed by means of a flow transducer, it is thus possible, for example in addition to the at least one, practically directly registered, flow parameter serving as primary measured variable, for example the volume flow, to ascertain, at least indirectly and, as a result, to measure, by means of the same measuring electronics, at least virtually, with application also of other, remotely registered, measured variables, for example, a remote, local temperature or a remote, local pressure in the medium, also derived, secondary measured variables, such as e.g. a mass flow and/or a density.
Experimental investigations on distributed measuring systems of the type being discussed, which, as shown e.g. also in U.S. Pat. No. 6,651,512, ascertain, by means of a directly measured, volume flow and a virtually measured density, a mass flow as an indirectly measured variable, have shown that, especially also despite application of internally, as well as externally, ascertained, measured variables proved to be very precise in the measuring ranges usual for the pertinent caliber of the process line, significant errors can arise in the result of a measurement virtual in the above sense. It is quite possible for these errors to lie in the range of about 5% of the actual measured variable or even beyond. This is the case, especially also when ascertaining measured variables, such as e.g. volume flow, temperature or pressure, as intermediate, really measured variables, and/or density as an intermediate variable measured virtually according to measuring and calculating methods recommended in the aforementioned industrial standards.
Further, comparative investigations have, in such case, additionally shown that the aforementioned measurement errors can show, among other things, a certain dependence on the instantaneous Reynolds number of the flow, as well as also on the instantaneous thermodynamic state of the medium. However, it has also been found, in this connection, that, in numerous industrial applications, especially those involving compressible and/or at least 2-phase media, the Reynolds number, or the thermodynamic state of the medium, can be not only chronologically but also spatially variable to a high degree, especially in the direction of the flow axis of the measuring system. Besides applications having at least partially compressible media, additionally especially also applications show a significant transverse sensitivity to spatial variances of the Reynolds number, or the thermodynamic state, when the measurement of at least one of the measured variables occurs at a measuring point (real or virtual), at which the process line has a caliber deviating at least from one of the measuring points (real or virtual) to the other. This is e.g. the case in the application of flow conditioners reducing the cross section of the line (such as in the case of e.g. nozzles serving as so-called reducers), which can find application in the inlet region of flow measuring transducers, or also in the application of flow conditioners increasing the cross section of the line (so-called diffusers) in the outlet region of flow measuring transducers. Measuring systems with such reducers and/or diffusers are described, for example, in GB-A 21 42 725, U.S. Pat. No. 5,808,209, US-A 2005/0092101, U.S. Pat. Nos. 6,880,410, 6,644,132, 6,053,054, 6,644,132, 5,052,229 or 6,513,393 and are used, for example, for improving accuracy of measurement of flow measuring transducers. It has, in such case, been further ascertained that such transverse sensitivities based on application of reducers and/or diffusers are significant for caliber ratios between about 0.6 and 0.7, while their influence for caliber ratios with extreme diameter jumps of smaller than 0.2 are quite negligible.
Another application area having a significant sensitivity to the aforementioned variances as affecting the desired accuracy of measurement concerns, furthermore, those measuring systems, which are provided for the flow measurement of heavy gases, such as, perhaps, carbon dioxide or also phosgene, or long-chain carbon compounds having a molecular wa of over 30 g/mol.
The above-described spatial variance of the Reynolds number can, in turn, lead to the fact that practically each of the aforementioned, mutually spaced, real measuring points of the distributed measuring system has, during operation, a local Reynolds number deviating, to a considerable degree, from the local Reynolds number of each of the other, also-used, measuring points. Equally, also the mentioned variance of the thermodynamic state would lead to the fact that mutually spaced, measuring points of the distributed measuring system can have thermodynamic states differing from one another. In view of this, thus, each of the measured variables, as measured on a distributed basis, would have to be adjusted according to the particularly associated, local Reynolds number and/or the particularly associated, local thermodynamic state, a task which, in the absence of the information required therefor, namely the, in each case, other, but remotely measured, state variables, is not directly possible. If, for example, the density and/or the mass flow, calculated on the basis of the measured state variables pressure and temperature, would be calculated without taking into consideration the variance of the Reynolds number, or thermodynamic state, an additional measurement error would result, having essentially a quadratic dependence on the flow velocity. Accordingly, for the aforementioned configuration, at flow velocities of clearly less than 10 m/s, the measuring accuracy of about 0.1% to 0.5%, currently strived for, is practically no longer significant.